The present invention relates to a press section of a machine for the manufacture of a fiber web, and particularly a paper web. In particular, the invention relates to the press nips and to the press belt which moves through the two press nips which are formed in a press having a single central roll. Such a press section of a paper making machine is comprised of a central roll, a first press roll at one side and a second press roll generally at the opposite side of the central roll, wherein the press rolls respectively define first and second press nips. A substantially water impervious, endless press belt travels through the first press nip, wraps around the central roll, travels through the second press nip and leaves the central roll. An endless felt belt guides the web to be dewatered through the first press nip and is separated from the web after the first press nip. A second endless felt belt may be provided for the second press nip and it operates substantially in the same manner as the first felt belt. The press belt conducts the paper web to be dewatered through the first press nip around the central roll through the second press nip and then carries the web to a place where further transport means, usually connected with a dryer section of the machine, removes the web from the press belt.
These features of a press section are known from U.S. Pat. No. 4,359,827. That known press section is formed of a set of four or five rolls which include a central roll 50 which cooperates with two or three press rolls disposed at locations around the central roll to form a first press nip 14 and a second press nip 17. A press belt 15 travels through these two press nips. It travels onto the central roll 50 in front of or upstream of the first press nip 14 and leaves the central roll behind or downstream of the second press nip 17. A first endless felt belt 12 supports the web W, which is to be dewatered, on its path from the preceding wire section of the machine, first through a pre-press nip 13, and from there into the first press nip 14. The first felt belt 12 is separated from the web W directly after passing through the first press nip 14. From this place, the press belt 15 conducts the web W around the central roll 50. Possibly, the press belt and the web pass through an additional press nip 16. The belt 15 then conducts the web into the second press nip 17. After that nip, the press belt conducts the web to a web removal point at which a dryer wire belt 19 picks up the web with the aid of a removal suction roll 18 and transfers the web into a following dryer section.
One object of the invention in the above noted U.S. patent is to provide a paper making machine having no free and unsupported length of paper web, i.e. no open draw, anywhere between the wire screen section upstream of the press section and the dryer section downstream of the press section and, therefore, in particular also through the press section. This avoids the paper web being weakened along a free length of the paper web while it is still wet and therefore has only little strength. The U.S. patent says to expect that higher operating speeds, fewer breaks or tears of paper, and the use of cheaper raw materials will be possible.
The U.S. patent discloses that the permeability and porosity of the second press belt 15, are slight, probably meaning as compared with those features of a felt belt. However, no further information about this is given in the U.S. patent.
One disadvantage of the press section of U.S. Pat. No. 4,359,827 is that it is difficult to detach the web from the press belt following the second press nip 17 and to reliably transfer the web to the dryer wire 19. It may be attempted to overcome these difficulties by increasing the vacuum prevailing in the removal suction roll 18, but with only slight success. There is a complicating circumstance that detachment of the web from the press belt is more difficult as the operating speed of the paper making machine increases.
U.S. Pat. No. 4,483,745 also describes different press sections in which a non-porous, water impervious press belt having a smooth surface travels through one or two of the press nips together with the web to be dewatered. Some of the embodiments are again disclosed as permitting no undesired open length of paper or open draw anywhere. However, the same problems will arise with regard to detachment of the web from the press belt as are present in an embodiment according to U.S. Pat. No. 4,359,827.